What causes a "bumpy" dyno line?
Thread Starter
Burning Brakes
Joined: Jun 2000
Posts: 1,112
Likes: 95
From: Blue Springs MO
What causes a "bumpy" dyno line?
I have a dyno from a 99 Camaro that the HP and torque lines are as smooth as glass. I recently had a 2001 Z06 dynoed and the torque lines are real bumpy from the start. The HP lines are fairly smooth until about 5400 RPMs then they get a little bumpy. What could be causing this in the Z06? Is there a problem with the car that I should look into? BTW, the dyno runs were at the same shop but on different days so I doubt that it is the equipment.
I did an Ease trace during the dyno run. I had 0.6 and 1.1 knock retard and no misfire. My timing was 25.5 degrees. It was showing that I was rich at WOT which I have corrected using LS1-Edit.
Any ideas what could cause this?
I did an Ease trace during the dyno run. I had 0.6 and 1.1 knock retard and no misfire. My timing was 25.5 degrees. It was showing that I was rich at WOT which I have corrected using LS1-Edit.
Any ideas what could cause this?
Safety Car
Joined: Nov 1999
Posts: 3,891
Likes: 2
From: Houston, TX
Re: What causes a "bumpy" dyno line? (Jim Shearer)
Stock platinum plugs that are missing pucks possibly?!?!
Bad wire???
Start there. If those two things do not take care of the problem, drop me a line and there are a few more things to look at possibly...
Good LUck,
Bad wire???
Start there. If those two things do not take care of the problem, drop me a line and there are a few more things to look at possibly...
Good LUck,
Safety Car
Joined: Aug 2000
Posts: 4,154
Likes: 1
From: Murphy TX
Cruise-In II Veteran
Re: What causes a "bumpy" dyno line? (Jim Shearer)
You need to find out if it's an artifact of the graph or the data. You can make noisy data appear better by smoothing (moving window averaging.) You can ususally "find" a couple extra HP from the peaks you get using less smoothing. If you could get the raw data for both and download viewers (I know DynoJet has a freebie viewer for dl), you could get a better feel for the underlying data.
If you can verify that the graphs used the same smoothing factor (and I'm assuming the same software overall) you could then assume the differences in the appearance of the graphs were meaningful.
If you can verify that the graphs used the same smoothing factor (and I'm assuming the same software overall) you could then assume the differences in the appearance of the graphs were meaningful.
Le Mans Master
Joined: Aug 2000
Posts: 5,531
Likes: 1,944
From: Austin Texas
Re: What causes a "bumpy" dyno line? (Jim Shearer)
A dynomometer measures the acceleration of a drum by measueing the amount of time it takes for a (fraction of a) revolution. The computer has a fixed minimum time accuracy limit that causes "granulation" of the readings. This is a kind of digital sampling noise, either a revolution occurs before one time interval, or before the next. Since there is no way to measure exactly when the event occured, just later than one fixed point and before the next fixed point. This causes an inaccuracy in the resulting TQ and HP plots that looks like noise that gets worse as the RPMs increase.
In actuality, the noise gets worse as the TQ and HP increase (not RPMs increase)--because there is successivly less and less time between measurements and therefore, larger time domain sampling errors. A 600 HP engine reads more noise than a 300 HP engine because its accelerating the drum proportionately faster. An engine that makes 300 HP at 5,000 RPMs has the same amount of noise as a different engine that makes 300 HP at 10,000 RPMs.
In actuality, the noise gets worse as the TQ and HP increase (not RPMs increase)--because there is successivly less and less time between measurements and therefore, larger time domain sampling errors. A 600 HP engine reads more noise than a 300 HP engine because its accelerating the drum proportionately faster. An engine that makes 300 HP at 5,000 RPMs has the same amount of noise as a different engine that makes 300 HP at 10,000 RPMs.
Thread Starter
Burning Brakes
Joined: Jun 2000
Posts: 1,112
Likes: 95
From: Blue Springs MO
Re: What causes a "bumpy" dyno line? (MitchAlsup)
Mitch,
Thanks for the great explanation of how the dyno works. However, I don't think this is the reason. I have a dyno for a 99 Camaro 6 speed that was done at the same shop by the same tech. The CAmaro had 326 HP and the Z06 had 341 HP. The Camaro's dyno lines are totally smooth.
I think there has to be another explanation.
Thanks for the great explanation of how the dyno works. However, I don't think this is the reason. I have a dyno for a 99 Camaro 6 speed that was done at the same shop by the same tech. The CAmaro had 326 HP and the Z06 had 341 HP. The Camaro's dyno lines are totally smooth.
I think there has to be another explanation.
Race Director
Joined: Aug 1999
Posts: 15,466
Likes: 0
From: GARDENA Ca, State Of Konfusion
St. Jude Donor '05
Re: What causes a "bumpy" dyno line? (Jim Shearer)
I changed my plugs to TR-55 and the stock wires to MSD's and that cleared up my erratic curve (jagged curve).
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Drifting
Joined: Oct 2000
Posts: 1,598
Likes: 1
From: Plano TX
Re: What causes a "bumpy" dyno line? (wallstAL)
One possibility that I've seen many times while doing dyno tuning as KR kicks in and causes the "bumps" up at the top end. Post or email me a scan of your dyno (with numbers if possible) and I'll be glad to look at it. Also, was the dyno done with wide band o2?
Melting Slicks
Joined: Feb 2001
Posts: 2,485
Likes: 0
From: Pembroke Pines Florida
Re: What causes a "bumpy" dyno line? (K&B Motorsports)
I had a Bumpy Power curve also. The Tuner said it is caused by not driving the car hard enough on a more frequent basis and said that a "Vortex" will also help. There are two lines from different days. After the first very bumpy pull, I changed my driving habits to more "redline shifts" as per Tuners advice. Then after a couple months the second dyno results showed a smoother but still bumpy line. The car seems to run very good. Now I have a Blackwing and Stingers and feel more power. I will dyno soon to see how the two mods affected my curve.
